The gun thought to be used in the Fort Hood massacre packs so much firepower, it's known as 'the Cop Killer,' federal law enforcement officials said.

Major Nidal Malik Hasan allegedly packed a FN Herstal Five-seveN tactical pistol, which according to federal law enforcement officials, was legally purchased from the "Guns Galore" shop in Killeen, Texas in Aug. 2009. The store's manager, David Cheadle, said that particular firearm can hold 20 rounds in a standard clip and take a ten round clip extension. Cheadle said with one clip and one round in the chamber, one could fire 31 rounds before reloading.

Hasan may have used an expanded clip in the shooting.

On FN Herstal's webpage, the benefits of the Five-seveN pistol note that it can "defeat the enemy in all close combat situations in urban areas, jungle conditions, night missions and any self defense action."

The second gun he had with him was a .357 S&W Magnum revolver, federal law enforcement officials tell ABC News. Ballistics are still being run to determine if he used the revolver in the shooting.

Federal agents, military investigators and Texas Rangers had been investigating whether the gun allegedly used by Hasan was purchased at "Guns Galore," Cheadle told ABC News.

"They called last night and asked about a specific name and gun purchase, but the name was not Hasan," said Cheadle.

Three investigators, including one wearing a Texas Rangers badge, were seen entering the store just before noon Friday. The manager said they were interviewing all of the store employees.

They declined to comment as they left Guns Galore in a car with Department of Defense markings.

The manager said he recognized a photo shown to him of Major Nidal Malik Hasan, identified by authorities as the suspect shooter at Fort Hood. But he said Hasan would have had to show identification if he had bought the gun in his name.

Cheadle said the agents were interested in a FN pistol that uses 5.7 caliber ammunition.